The present invention relates to superconducting magnetic energy storage devices, particularly in the form of solenoid coils which are installed in annular enclosures, possibly below ground.
The unique electrical properties of superconductors have led to proposals for storing large quantities of electrical energy in large superconducting coils, one type of which would be an annular solenoid installed, possibly below ground, so that its axis is vertical. The proposed solenoids would have a substantial axial height and diameter, the diameter possibly being of the order of 500 to 1000 meters or more.
The superconductive materials which are currently usable in practice, such as NbTi, must be maintained at a temperature in the vicinity of 1.degree.-4.degree. K. in order to exhibit superconducting properties. Such temperatures can be established by surrounding the superconducting material with successive envelopes of cryogens having progressively higher boiling points.
For example, it is known to use, for this purpose, an envelope of helium, which can be made to have a boiling point lower than 4.degree. K., surrounded by an envelope of neon, having a boiling point of in the vicinity of 28.degree. K., the latter being surrounded by an envelope of nitrogen, having a boiling point in the vicinity of 77.degree. K. Each of these cryogens is maintained in the liquid state by a suitable refrigeration system.
During cooldown of such a coil from room temperature to superconducting temperatures, and during subsequent operation in which cyclically varying radial forces are produced by the magnetic fields associated with the coil, the coil conductors will be subjected to stresses in the direction of their length. If these stresses produce even low levels of strain, the conductivity of the stabilizer associated with the conductor will be degraded, and this will degrade the performance of the conductor.
In addition, known structures for supporting such conductors enable the conductors to contact liquid helium only over a limited portion of their circumference, typically of the order of a quarter of their circumference. This makes it difficult to maintain the entire conductor at its superconducting temperature, particularly when conditions occur which cause the cryogen in contact with the conductors to experience a transition from the super fluid state to a two-phase boiling heat transfer condition.
Furthermore, the coil support structures which have already been proposed attempt to deal with the problems of radial contraction stresses during cooldown to superconductive temperatures in ways which cause slipping movements which generate heat due to friction and/or which induce high bending stresses and/or which require the creation of a loose coil pack which will experience insulation abrasion and movement during operation of the coil.